Method of using cyclodextrin-based metal organic frameworks

ABSTRACT

This disclosure relates to a method that includes (1) contacting a solvent with a porous cyclodextrin-based metal organic framework (CD-MOF) adsorbed with CO 2  to release CO 2 , and (2) collecting the released CO 2 . The CD-MOF includes at least a metal cation and a plurality of cyclodextrin components.

CROSS REFERENCE TO RELATED APPLICATION

Pursuant to 35 U.S.C. §119(e), this application claims priority to U.S. Provisional Application Ser. No. 62/198,407, filed Jul. 29, 2015, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to methods of using cyclodextrin-based metal organic frameworks (CD-MOFs), as well as related systems, components, and products.

BACKGROUND

Sequestration of carbon dioxide from gaseous waste streams has become a pressing issue for the scientific and global community in light of the predicted detrimental effects of anthropogenic CO₂ production. Recently, several approaches toward this goal have emerged using metal organic frameworks (MOFs) derived from petrochemical sources. MOFs, also known as coordination polymers or coordination networks, are typically composed of a metal center coordinated to organic linkers to form highly-porous and well-defined crystalline structures that often have much higher surface areas compared to the conventional activated carbon and zeolites. Free hydroxyl and amine residues are known to react with carbon dioxide to form carbonic acids and carbamic acids, respectively. These functionalities have been added to MOFs by rational design of structures. While these advances are noteworthy in their incremental storage capacity, they are generally synthesized from environmentally malevolent materials. Thus, there remains a need to develop functional materials from simple components that are renewable and biocompatible.

SUMMARY

This disclosure is based on the unexpected discovery that CO₂ can be readily released from a CD-MOF by contacting a solvent (e.g., an organic solvent, water, or a mixture thereof) with a CD-MOF adsorbed with CO₂. The released CO₂ can then be collected for use in various applications.

In one aspect, this disclosure features a method that includes (1) contacting a solvent with a porous cyclodextrin-based metal organic framework (CD-MOF) adsorbed with CO₂ to release CO₂, and (2) collecting the released CO₂. The CD-MOF includes at least a metal cation and a plurality of cyclodextrin components.

In another aspect, this disclosure features a method that includes (1) disposing a porous CD-MOF in a gas comprising at least about 0.04% by volume of CO₂ to form a CD-MOF adsorbed with CO₂, and (2) contacting a solvent with the CD-MOF adsorbed with CO₂ to release CO₂. The CD-MOF includes at least a metal cation and a plurality of cyclodextrin components.

In still another aspect, this disclosure features a method that includes (1) contacting a solvent with a porous CD-MOF adsorbed with CO₂ to release CO₂, and (2) collecting the released CO₂. The CD-MOF includes at least a metal cation and a plurality of cyclodextrin components. The CD-MOF adsorbed with CO₂ includes at least about 4% by weight of CO₂.

Embodiments can include one or more of the following features.

In some embodiments, the CD-MOF adsorbed with CO₂ can include at least about 4% by weight and/or at most about 10% by weight of CO₂.

In some embodiments, the solvent can be C₁₋₁₀ alcohol, C₁₋₁₀ alkane, methylene chloride, water, acetone, acetic acid, acetonitrile, benzene, toluene, dimethylformamide, or a mixture thereof.

In some embodiments, the solvent can be saturated with CO₂.

In some embodiments, the solvent can be in the form of a liquid or a vapor.

In some embodiments, the cyclodextrin can be γ-cyclodextrin.

In some embodiments, the metal cation can be a Group I metal cation (e.g., Na⁺, K⁺, Rb⁺, or Cs⁺), Group II metal cation, or a transition metal cation.

In some embodiments, the method can further include disposing a CD-MOF in a gas comprising at least about 0.04% by volume and/or at most about 25% by volume of CO₂ to form the CD-MOF adsorbed with CO₂.

In some embodiments, the method can further include transferring the CD-MOF adsorbed with CO₂ into a regeneration vessel prior to the contacting step.

In some embodiments, the method can further include removing ambient air from the regeneration vessel comprising the CD-MOF adsorbed with CO₂ prior to the contacting step.

In some embodiments, the contacting step can include supplying the solvent to the regeneration vessel containing the CD-MOF adsorbed with CO₂.

In some embodiments, the collecting step can include transferring the CO₂ released from the contacting step from the regeneration vessel to a condensation vessel.

In some embodiments, the collecting step can further include cooling the CO₂ in the condensation vessel or the CO₂ released from the contacting step to liquefy the vapor of the solvent in the CO₂, and removing the solvent to purify the CO₂.

In some embodiments, the collecting step can further include transferring the CO₂ from the condensation vessel to a collection vessel by using a compressor.

Other features, objects, and advantages of the invention will be apparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1A illustrates the structure of α-1,4-linked D-glucopyranosyl residue.

FIG. 1B illustrates the structure of a γ-cyclodextrin ring.

FIG. 1C illustrates the eight monosaccharide residues in γ-CD form a truncated cone with the C6 hydroxy (OH) groups constituting the primary (1°) face and the C2 and C3 OH groups constituting the secondary (2°) face.

FIG. 2A illustrates a 3D representation of a CD-MOF having a (γ-CD)₆ cube. The primary faces of the γ-CD rings point inwards and secondary faces point outward.

FIG. 2B illustrates a CD-MOF body-centered cubic packing arrangement containing nine (γ-CD)₆ cubes.

FIG. 2C illustrates the CD-MOF shown in FIG. 2B in which the pores within the (γ-CD)₆ cubes are shown as cut-outs.

FIG. 2D illustrates the structural formula of the CD-MOF unit shown in FIGS. 2B and 2C that contains the alternating coordination of M+ ions to 1) the primary face C6 OH groups and glycosidic ring oxygen atoms and ii) secondary face C2 and C3 OH groups.

FIG. 3 shows different phases for CO₂ binding activity within a CD-MOF porous structure when the CD-MOF is exposed to a gas containing up to 1% by volume CO₂ at ambient temperatures and pressures. The amount of CO₂ shown in FIG. 3 refers to the percent mass CO₂ per unit mass of CD-MOF.

FIG. 4 shows an exemplary regeneration process to release CO₂ from CD-MOF adsorbent by using a solvent.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This disclosure generally relates to methods of releasing or collecting CO₂ from a CD-MOF by contacting a CD-MOF adsorbed with CO₂ with a solvent (e.g., an organic solvent, water, or a mixture thereof).

CD-MOFs

In general, the CD-MOFs that can be used in the methods described herein can be those described in U.S. Pat. No. 9,085,460, the contents of which are hereby incorporated by reference in their entirely.

The CD-MOFs generally include at least one metal cation (e.g., a plurality of metal cations) and a plurality of cyclodextrin components (such as those of Formula (I) below). The at least one metal cation is generally coordinated with the plurality of cyclodextrin molecules or cyclodextrin derivatives. In general, the CD-MOFs are porous.

Suitable metal cations that can be used in the CD-MOFs include Group I metal cations (e.g., Na⁺, K⁺, Rb⁺, or Cs⁺), Group II metal cations (e.g., Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺), and transition metal cations (e.g., Mn⁴⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺). The metal cations can be included into the CD-MOFs by using salts or bases as starting materials. Examples of suitable salts include KF, KCl, KBr, K₂CO₃, K₂(azobenzene-4,4′-dicarboxylate), Na₂CO₃, and NaBPh₄. Examples of suitable bases include KOH, NaOH, RbOH, and CsOH.

In general, the main building block for CD-MOFs is cyclodextrin (CD), a cyclic oligosaccharide that includes monosaccharide residues linked in a circular ring. Suitable cyclodextrins that can be used in the CD-MOFs include, for example, α-, β- and γ-cyclodextrins. FIG. 1A illustrates the structure of α-1,4-linked D-glucopyranosyl residue. FIG. 1B illustrates the structure of a γ-cyclodextrin ring. FIG. 1C illustrates the eight monosaccharide residues in γ-CD form a truncated cone with the C6 hydroxy (OH) groups constituting the primary (1°) face and the C2 and C3 OH groups constituting the secondary (2°) face. Cyclodextrins can be mass-produced through enzymatic degradation of a renewable source (e.g., starch). In some embodiments, a CD-MOF can be made from one or more cyclodextrin derivatives (such as those shown in Formula (I) below).

Generally, CD-MOFs can be prepared by dissolution of both the cyclodextrin component (e.g., γ-cyclodextrin) and the metal-containing component (such as a metal salt (e.g., KCl) or a base containing a metal cation (e.g., KOH)) in a solvent (e.g., water) in which both have solubility. Isolation of CD-MOFs can be achieved by addition of a poor solvent in which either of the above components has poor solubility. Suitable poor solvents, includes C₁-C₁₈ alcohols, acetone, tetrahydrofuran, dioxane, acetonitrile, or a mixture thereof.

In some embodiments, CD-MOFs can be prepared by the following method. At ambient temperatures and pressures, γ-CD can be dissolved in an aqueous solution containing an alkali metal cation (e.g., K⁺), and followed by vapor diffusion of a water-miscible solvent (e.g., methanol) to form millimeter-sized body-centered cubic crystalline structures. Without wishing to be bound by theory, it is believed that the γ-CD rings adopt the faces of a cube, with their primary (1°) faces pointing towards the interior of the cube and their secondary (2°) faces pointing outward. Further, without wishing to be bound by theory, it is believed that the γ-CD rings are linked together by coordination of the alkali metal cations to the primary C6 OH groups and the glycosidic ring oxygen atoms. The individual cubes pack to form the body-centered cubic crystal through coordination of more alkali metal cations to the C2 and C3 OH groups of the secondary faces of the γ-CD rings. FIG. 2A-2D illustrate the CD-MOF geometry with a coordinating alkali metal. Specifically, FIG. 2A illustrates a 3D representation of a CD-MOF having a (γ-CD)₆ cube. The primary faces of the γ-CD rings point inwards and secondary faces point outward. FIG. 2B illustrates a CD-MOF body-centered cubic packing arrangement containing nine (γ-CD)₆ cubes. FIG. 2C illustrates the CD-MOF shown in FIG. 2B in which the pores within the (γ-CD)₆ cubes are shown as cut-outs. FIG. 2D illustrates the structural formula of the CD-MOF unit showing in FIGS. 2B and 2C that contains the alternating coordination of M⁺ ions to i) the primary face C6 OH groups and glycosidic ring oxygen atoms and ii) secondary face C2 and C3 OH groups.

In some embodiments, the CD-MOFs described herein includes a CD component and a metal-containing component. The metal-containing component can have the formula MN. M can be a Group I, Group II metal or transition metal, and N can be an organic or inorganic, monovalent or multivalent anion. Suitable inorganic anions include, for example, chloride, fluoride, hydroxide, sulfide, sulfinate, carbonate, chromate, and cyanide. Suitable organic anions include, for example, benzoate, azobenzene-4,4′-dicarboxylate, acetate, and oxalate. The CD component of the CD-MOFs can be a compound of the Formula (I):

in which n=0-10; R is selected from the group consisting of —OH; —NR′R″; C₁-C₁₈ alkyl optionally substituted with one, two, three, four or five R₁ groups; C₂-C₁₈ alkenyl optionally substituted with one, two, three, four or five R₁ groups; C₂-C₁₈ alkynyl optionally substituted with one, two, three, four or five R₁ groups; C₁-C₁₈ alkoxy optionally substituted with one, two, three, four or five R₁ groups; —S(═O)₂R′; —S(═O)OR′; —S(═O)R′; —C(═O)OR′; —CN; —C(═O)R′; —SR′, —N═N⁺═N⁻; —NO₂, —OSO₂R′; —C(═O)OR′; —O(═S)SR′; —P(═O)(OR′)₂; —OP(═O)(OR′)₂; —P(═O)(OR′)R″; —N═R′R″; —NR′P(OR″)(OR′″); —OC(═O)NR′R″; aryl optionally substituted with one, two, three, four or five R₂ groups; heteroaryl optionally substituted with one, two, three, four or five groups independently selected from R₂ groups; and cycloalkyl optionally substituted with one, two, three, four or five groups independently selected from R₂ groups; each R₁ group is independently selected from the group consisting of hydroxyl, halo, C₁-C₆ alkoxy, —NR′R″; —S(═O)₂R′; —S(═O)OR′; —S(═O)R′; —C(═O)OR′; —CN; —C(═O)R′; —SR′, —N═N⁺═N⁻; −NO₂, —OSO₂R′; —C(═O)OR′; —O(═S)SR′; —P(═O)(OR′)₂; —OP(═O)(OR′)₂; —P(═O)(OR′)R″; —N═R′R″; —NR′P(OR″)(OR′″); —OC(═O)NR′R″; aryl optionally substituted with one, two, three, four or five R′ groups; heteroaryl optionally substituted with one, two, three, four or five groups independently selected from R′ groups; and cycloalkyl optionally substituted with one, two, three, four or five groups independently selected from R′ groups; each R₂ group is independently selected from the group consisting of C₁-C₆ alkyl, C₂-C₆ alkenyl, C₂-C₆ alkynyl, hydroxyl, halo, C₁-C₆ alkoxy, —NR′R″; —S(═O)₂R′; —S(═O)OR′; —S(═O)R′; —C(═O)OR′; —CN; —C(═O)R′; —SR′, —N═N⁺═N⁻; —NO₂, —OSO₂R′; —C(═O)OR′; —O(═S)SR′; —P(═O)(OR′)₂; —OP(═O)(OR′)₂; —P(═O)(OR′)R″; —N═R′R″; —NR′P(OR″)(OR′″); —OC(═O)NR′R″; aryl optionally substituted with one, two, three, four or five R′ groups; heteroaryl optionally substituted with one, two, three, four or five groups independently selected from R′ groups; and cycloalkyl optionally substituted with one, two, three, four or five groups independently selected from R′ groups; and wherein each R′, R″, and R′″ are independently selected from the group consisting of H, C₁-C₆ alkyl, and aryl. Examples of compounds of Formula (I) include α-, β- and γ-cyclodextrins.

As used herein, the term “alkyl” refers to a straight or branched chain alkyl radical. Examples include, but are not limited, to methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, 2-pentyl, isopentyl, neopentyl, hexyl, 2-hexyl, 3-hexyl, and 3-methylpentyl. Each alkyl group may be optionally substituted with one, two or three substituents such as a halo, cycloalkyl, aryl, alkenyl or alkoxy group.

As used herein, the term “alkenyl” refers to a straight or branched hydrocarbon radical having one or two double bonds and includes, for example, ethenyl, propenyl, 1-but-3-enyl, 1-pent-3-enyl, and 1-hex-5-enyl. The alkenyl group can also be optionally mono-, di-, or trisubstituted with, for example, halo, aryl, cycloalkyl or alkoxy.

As used herein, the term “alkynyl” refers to a straight or branched hydrocarbon radical having one or two triple bonds and includes, for example, propynyl and 1-but-3-ynyl. The alkynyl group can also be optionally mono-, di-, or trisubstituted with, for example, halo, aryl, cycloalkyl or alkoxy.

As used herein, the term “alkoxy” refers to an —O-alkyl group in which the alkyl is as defined above.

As used herein, the term “halo” or “halogen” refers to a halogen radical of fluorine, chlorine, bromine, or iodine.

As used herein, the term “aryl” refers to an aromatic carbocylic radical having a single ring (e.g. phenyl), multiple rings (e.g. biphenyl), or multiple fused rings in which at least one is aromatic (e.g. 1,2,3,4-tetrahydronaphthyl).

As used herein, the term “heteroaryl” refers to one aromatic ring or multiple fused aromatic ring systems of 5-, 6- or 7-membered rings containing at least one and up to four heteroatoms (e.g., nitrogen, oxygen or sulfur). Examples include, but are not limited to, furanyl, thienyl, pyridinyl, pyrimidinyl, benzimidazolyl and benzoxazolyl.

As used herein, the term “cycloalkyl” refers to a carbocylic radical having a single ring (e.g., cyclohexyl), multiple rings (e.g., bicyclohexyl) or multiple fused rings (e.g., decahydronaphthalenyl). In addition, the cycloalkyl group may have one or more double bonds.

Adsorption of CO₂ by CD-MOFs

In general, the CD-MOFs described herein have a strong preference for CO₂ adsorption (e.g., through chemisorption or physisorption). For example, without wishing to be bound by theory, it is believed that CO₂ can be captured by the CD-MOFs as a result of chemisorption onto the uncoordinated hydroxyl groups on the CD rings (e.g., γ-CD rings).

The amount of CO₂ that can be adsorbed by the CD-MOF s described herein depend on various factors, such as the CO₂ concentration of the gas to which the CD-MOFs are exposed. In some embodiments, when the CD-MOF s are exposed to earth atmosphere (which contains about 0.038% by volume of CO₂), the CD-MOF s can adsorb about 3-4% by weight of CO₂ based on the unit weight of the CD-MOFs. In some embodiments, the CD-MOFs are exposed to a gas containing a CO₂ concentration higher than that in the earth atmosphere. Such a gas can include at least about 0.04% by volume (e.g., at least about 0.1% by volume, at least about 0.5% by volume, at least about 1% by volume, at least about 2% by volume, or at least about 5% by volume) and/or at most about 25% by volume (e.g., at most about 20% by volume, at most about 15% by volume, at most about 10% by volume, at most about 5% by volume, or at most about 1% by volume) of CO₂. In such embodiments, the CD-MOFs can adsorb at least about 4% by weight (e.g., at least about 4.5% by weight, at least about 5% by weight, at least about 5.5% by weight, or at least about 6% by weight) and/or at most about 10% by weight (e.g., at most about 9% by weight, at most about 8% by weight, at most about 7% by weight, at most about 6% by weight, or at most about 5% by weight) of CO₂, based on the unit weight of the CD-MOFs.

It is worth noting that individual CD rings generally do not exhibit any CO₂ capture property. Without wishing to be bound by theory, it is believed that, although the surface accessibility of the cyclodextrin hydroxyl groups in the CD-MOF crystal is much higher than in an amorphous cyclodextrin powder, the main reason for the high selective adsorption of CO₂ onto CD-MOFs is the broken symmetry and isolation of the OH groups that arises when the other OH groups are preoccupied holding the CD-MOF lattice together through coordination with metal cations. It is believed that the isolated, free hydroxyl groups can serve as reactive hotspots to adsorb CO₂ by reversibly binding to the CO₂ to form carbonic acid groups.

Without wishing to be bound by theory, it is believed that, the above-mentioned free hydroxyl groups can be fixed in a rigid and highly porous crystalline scaffold, which facilitates easy diffusion of CO₂ within the crystal lattice such that CO₂ can readily react and bind to the free OH groups. In addition, it is believed that CD-MOFs described herein can be very selective for adsorption of CO₂ at low CO₂ partial pressures or low CO₂ concentration (e.g., in a gas containing less than 1% by volume CO₂).

Without wishing to be bound by theory, it is believed that the CD-MOFs described herein can have three phases for CO₂ adsorption. FIG. 3 shows different phases for CO₂ binding activity within a CD-MOF porous structure when the CD-MOF is exposed to a surrounding gas containing up to 1% by volume CO₂ at ambient temperatures and pressures. The amounts of CO₂ shown in FIG. 3 refer to the weight percent CO₂ per unit weight of the CD-MOF. As shown in FIG. 3, when a CD-MOF is exposed to a gas containing 1% by volume CO₂, the first adsorption phase is the initial about 0-0.8 wt % of CO₂ adsorbed onto the CD-MOF, which is believed to be bound with strong chemisorption. The second phase is a weak chemisorption of about 0.8-4 wt % of CO₂ adsorbed onto the less reactive OH groups in the CD-MOF. It is believed that the CO₂ strongly or weakly chemisorbed onto a CD-MOF is not easily released by lowering pressure alone (i.e., a low energy method). The third phase is a weaker physisorption of about 4-6 wt % of CO₂, which can be released relatively easily by a reduction in pressure.

According to FIG. 3, about 53% of the CO₂ adsorbed from a gas environment having a low CO₂ concentration (e.g., less than 1% by volume) is through a weak chemisorption. Without wishing to be bound by theory, it is believed that an advantage of a desorption method described below is that it can readily release the CO₂ adsorbed onto a CD-MOF through weak chemisorption by using simple steps at a low cost. By contrast, a conventional method either is not able to release the CO₂ adsorbed through weak chemisorption (e.g., by pressure reduction) or requires a large consumption of energy to release the CO₂ adsorbed through weak chemisorption (e.g., by increasing temperature). As a result, the desorption methods described herein can be advantageously used to release the CO₂ adsorbed through weak chemisorption to collect CO₂ from a gas environment having a low CO₂ concentration (e.g., the gas environment near a coal-based emission source).

Desorption of CO₂ from CD-MOFs

In general, one can use the desorption methods described herein to release the CO₂ adsorbed onto CD-MOFs. In some embodiments, the methods include (1) contacting a solvent with a porous CD-MOF adsorbed with CO₂ to release CO₂; and (2) collecting the released CO₂. The CD-MOF can be those described above.

In some embodiments, the solvent can be an organic solvent, an inorganic solvent, or a mixture thereof. Examples of suitable organic solvents include C₁₋₁₀ alcohol, C₁₋₁₀ alkane, methylene chloride, acetone, acetic acid, acetonitrile, benzene, toluene, dimethylformamide, or a mixture thereof. Examples of suitable inorganic solvents include water, and aqueous solutions (e.g., those containing one or more organic or inorganic solutes).

In some embodiments, the solvent used the desorption methods described herein can be in the form of a liquid or a vapor. When the solvent is in the form of a vapor, the vapor can be obtained from a liquid solvent (e.g., by heating or reduced pressure) and brought into contact with the CD-MOF.

In some embodiments, the solvent used the desorption methods described herein can be saturated with CO₂. Without wishing to be bound by theory, it is believed that the solvent releases CO₂ from the CD-MOF adsorbed with CO₂ by a mechanism (e.g., disrupting the binding of CO₂ with the CD-MOF) in addition to dissolving CO₂ in the solvent. Without wishing to be bound by theory, it is believed that an advantage of using a solvent saturated with CO₂ is that it can be recycled for further use without the need to remove any CO₂ dissolved in the solvent, thereby significantly reducing the costs of collecting and storing CO₂.

In some embodiments, the desorption methods can further include a step of disposing a porous CD-MOF in a gas containing at least about 0.04% by volume of CO₂ (i.e., a gas having a CO₂ concentration higher than that in the earth atmosphere) to form a CD-MOF adsorbed with CO₂. Such a gas can be a waste gas from a conventional post-combustion carbon capture and storage (CCS) system, or a gas emitted from or surrounding a coal-based CO₂ emission source (e.g., a coal-based power plant). The CD-MOF can be included in an open container (e.g., a canister) to adsorb the CO₂ in the gas. In some embodiments, the gas to which the CD-MOF is exposed is at an ambient temperature and pressure.

In some embodiments, a pH indicator can be used to detect whether the CD-MOF has adsorbed a sufficient amount of CO₂ so that it is ready to be regenerated. The pH indicator can be embedded in the CD-MOF and can change color upon the formation of the carbonic acid when the CO₂ gas is adsorbed onto the CD-MOF crystal. Such a pH indicator can serve as an in-situ CO₂ sensor to indicate when the CD-MOF needs to be regenerated. As an example, methyl red, a zwitterionic azobenezene-based pH indicator, can be diffused into the pores of a CD-MOF during synthesis. The CD-MOF with the embedded methyl red pH indicator but without CO₂ has a yellow color due to the interstitial OH— counter ions within the CD-MOF that creates a basic environment. When CO₂ is captured and forms carbonic acid groups, an acidic environment is created, thereby changing the color of the CD-MOF to red. Once it is indicated that the adsorption is completed, the container containing the CD-MOF adsorbed with CO₂ can be sealed and transported to a facility to regenerate the CO₂ adsorbed by the CD-MOF.

In some embodiments, after the CD-MOF adsorbed with CO₂ is formed, it can be transferred into a regeneration vessel. Optionally, after the CD-MOF adsorbed with CO₂ is transferred into the regeneration vessel, the ambient air can be removed from the regeneration vessel, e.g., by reducing the pressure in the regeneration vessel. Without wishing to be bound by theory, it is believed that reducing the pressure in the regeneration vessel can remove the ambient air in the regeneration vessel and/or a certain amount of CO₂ adsorbed on the CD-MOF through physisorption, but does not substantially remove CO₂ adsorbed on the CD-MOF through chemisorption. Thus, it is believed that this optional step can significantly increase the purity of the CO₂ obtained from contacting the CD-MOF adsorbed with CO₂ with a solvent as the CO₂ thus obtained would not include a substantial amount of ambient air in the regeneration vessel.

In some embodiments, after the optional step of removing the ambient air in the regeneration vessel, a solvent can be transferred into the regeneration vessel to contact the CD-MOF adsorbed with CO₂, thereby regenerating the CO₂ from the CD-MOF. In such embodiments, the CO₂ released from the above step can be transferred out of the regeneration vessel into a condensation vessel.

In some embodiments, the condensation vessel can be cooled so that, once the CO₂ is transferred into the condensation vessel, the vapor of the solvent in the CO₂ can be liquefied and then removed to further purify the CO₂. The CO₂ thus obtained can then be transferred to a collection vessel using a compressor for storage purposes. The collected CO₂ can be used in various industrial or consumer applications (e.g., to make dry ice or soda).

In some embodiments, the CD-MOF and solvent used in the desorption methods described herein can be recycled without the need of further purification. For example, the CD-MOF can be directly reused in a gas having a higher CO₂ concentration than earth atmosphere even though it includes a residual amount of CO₂. As another example, as noted above, the solvent can be reused in the desorption methods described herein without the need to remove any CO₂ dissolved in the solvent.

It is believed that the methods described herein have relatively low costs at least because the CD-MOFs and solvents used the methods can be recycled easily. It is also believed that the methods described herein are environmental friendly at least because the CD-MOFs are obtained from renewable resources and are biodegradable. Further, it is believed that the methods described herein are particularly suitable for capturing CO₂ from a gas having a low CO₂ concentration (e.g., containing less than about 25% by volume or less than about 1% by volume of CO₂) because the CD-MOFs can adsorb a relatively large amount of CO₂ in such a gas and because these methods consume a relatively small amount of energy to capture and regenerate CO₂, thereby resulting in a relatively high energy efficiency. By contrast, a conventional method would either be impractical or would have a very low energy efficiency because they require use of a relative large amount of energy (e.g., by heating) to collect a relatively small amount of CO₂ (as the gas the adsorbent is exposed to has a low CO₂ concentration).

FIG. 4 illustrates an exemplary process for regenerating CO₂ from a CO₂-filled CD-MOF in containers. As shown in FIG. 4, once the CO₂-filled CD-MOF containers are transferred into a regeneration vessel, two steps (i.e., steps A and B) can occur. Step A is the evacuation of the regeneration vessel (e.g., by slightly lowering the pressure in the vessel) to remove the ambient air in the vessel, which may also include other gases such as N₂, O₂, and CH₄ (e.g., if the CD-MOF containers are taken from capture sites located near coal beds). For example, the pressure of the regeneration vessel can be lowered from ambient pressure (i.e., about 101 kPa) to about 10-100 kPa. As noted above, it is believed that step A may remove a certain amount of CO₂ adsorbed on the CD-MOFs through physisorption, but does not substantially remove CO₂ adsorbed on the CD-MOFs through chemisorption. The amount of the CO₂ in the gas removed in step A can be monitored by using gas chromatography.

In some embodiments, the CO₂ removed in step A can be recovered by contacting the gas obtained from step A with a fresh CD-MOF in a recycle/re-capture process. For example, the CO₂ removed in step A can be transferred into a condenser containing a freshly-made CD-MOF or a regenerated CD-MOF that does not include a substantial amount of CO₂. The CO₂ removed in step A can then be captured by the CD-MOF, which can be regenerated by the desorption methods described herein.

In some embodiments, step A can be performed before the CO₂-filled CD-MOF is transferred into the regeneration vessel. In some embodiments, step A is optional and can be omitted.

Step B is the introduction of a solvent (e.g., methanol), either as a liquid or through vapor diffusion, to release the weakly-chemisorbed CO₂ from CD-MOFs for collection. As shown in FIG. 3, the majority of adsorbed CO₂ in CD-MOFs is held by weak chemisorption. Thus, step B is the main step that regenerates most of the CO₂ from CO₂-filled CD-MOFs. The amount of the CO₂ in the gas exiting the regeneration vessel can be monitored by using gas chromatography. As shown in FIG. 4, the CO₂ exiting the regeneration vessel can be delivered to a condenser in which any solvent vapor in the CO₂ can be liquefied and removed. The CO₂ exiting the condenser can then be collected and stored. The regenerated CD-MOF can then be reused to capture CO₂ in a suitable gas.

In some embodiments, one can release and collect CO₂ by dissolving the CD-MOF adsorbed with CO₂ in water or an aqueous solution. In such embodiments, as the CD-MOF is dissolved and forms CD molecules, all of the CO₂ adsorbed by the CD-MOF is released. The released CO₂ can be dissolved in the water or aqueous solution. Once the water or aqueous solution is saturated with CO₂, any extra CO₂ can be released to the surround environment and collected. The CD dissolved in the water or aqueous solution can be recycled and reused to form a CD-MOF, e.g., by using the methods described herein.

For example, as shown in FIG. 4, a suitable amount of water can be added to the regeneration vessel to dissolve the CD-MOF adsorbed with CO₂. The amount of the water can be no more than the amount needed to dissolve the CD-MOF to reduce the amount of the CO₂ dissolved in the water. The CO₂ released from this step can be transferred from the regeneration vessel to a condenser (in which the water vapor in the CO₂ can be liquefied and removed), and subsequently to a collection vessel. The CD dissolved in the water in the regeneration vessel can be recycled and reused to form a CD-MOF.

Other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A method, comprising: contacting a solvent with a porous cyclodextrin metal organic framework (CD-MOF) adsorbed with CO₂ to release CO₂; and collecting the released CO₂, wherein the CD-MOF includes metal cations and a cyclodextrin, the cyclodextrin having the formula:

wherein n is an integer ranging from 0 to 10; R is an —OH group, and, the metal cations coordinate to a primary face C6 —OH group and glycosidic ring oxygen atom of glycosidic rings of the cyclodextrin, or, to secondary face C2 and C3 —OH groups of alternating glycosidic rings of the cyclodextrin.
 2. The method of claim 1, wherein the CD-MOF adsorbed with CO₂ comprises at least about 4% by weight of CO₂.
 3. The method of claim 1, wherein the CD-MOF adsorbed with CO₂ comprises at most about 10% by weight of CO₂.
 4. The method of claim 1, wherein the solvent is C₁₋₁₀ alcohol, C₁₋₁₀ alkane, methylene chloride, water, acetone, acetic acid, acetonitrile, benzene, toluene, dimethylformamide, or a mixture thereof.
 5. The method of claim 1, wherein the solvent is saturated with CO₂.
 6. The method of claim 1, wherein the solvent is in the form of a liquid or a vapor.
 7. The method of claim 1, wherein the cyclodextrin is γ-cyclodextrin.
 8. The method of claim 1, wherein the metal cations are Group I metal cations, Group II metal cations, or transition metal cations.
 9. The method of claim 8, wherein the CD-MOF further includes an anion wherein: the metal cations and the anion have a formula MN, where M is one of the metal cations and N is an organic or inorganic monovalent or multivalent anion.
 10. The method of claim 1, wherein, prior to the contacting step, the method further comprises disposing a CD-MOF in a gas comprising at least about 0.04% by volume of CO₂ to form the CD-MOF adsorbed with CO₂.
 11. The method of claim 10, wherein the gas comprises at most about 25% by volume of CO₂.
 12. The method of claim 10, further comprising transferring the CD-MOF adsorbed with CO₂ into a regeneration vessel prior to the contacting step.
 13. The method of claim 12, further comprising removing ambient air from the regeneration vessel comprising the CD-MOF adsorbed with CO₂ prior to the contact step.
 14. The method of claim 12, wherein the contacting step comprises supplying the solvent to the regeneration vessel comprising the CD-MOF adsorbed with CO₂.
 15. The method of claim 14, wherein the collecting step comprises transferring the CO₂ released from the contacting step from the regeneration vessel to a condensation vessel.
 16. The method of claim 15, wherein the collecting step further comprises cooling the CO₂ in the condensation vessel to liquefy the vapor of the solvent in the CO₂, and removing the solvent to purify the CO₂.
 17. The method of claim 16, wherein the collecting step further comprises transferring the CO₂ from the condensation vessel to a collection vessel by using a compressor.
 18. A method, comprising: disposing a porous cyclodextrin metal organic framework (CD-MOF) in a gas comprising at least about 0.04% by volume of CO₂ to form a CD-MOF adsorbed with CO₂; and contacting a solvent with the CD-MOF adsorbed with CO₂ to release CO₂, wherein the CD-MOF includes metal cations and a cyclodextrin, the cyclodextrin having the formula:

wherein n is an integer ranging from 0 to 10; R is an —OH group, and, the metal cations coordinate to a primary face C6 —OH group and glycosidic ring oxygen atom of glycosidic rings of the cyclodextrin, or, to secondary face C2 and C3 —OH groups of alternating glycosidic rings of the cyclodextrin.
 19. The method of claim 18, wherein the gas comprises at most about 25% by volume of CO₂.
 20. A method, comprising: contacting a solvent with a porous cyclodextrin metal organic framework (CD-MOF) adsorbed with CO₂ to release CO₂, the CD-MOF adsorbed with CO₂ comprises at least about 4% by weight of CO₂; and collecting the released CO₂, wherein the CD-MOF includes metal cations and a cyclodextrin, the cyclodextrin having the formula:

wherein n is an integer ranging from 0 to 10; R is an —OH group, and, the metal cations coordinate to a primary face C6 —OH group and glycosidic ring oxygen atom of glycosidic rings of the cyclodextrin, or, to secondary face C2 and C3 —OH groups of alternating glycosidic rings of the cyclodextrin.
 21. The method of claim 20, wherein the collecting step comprises cooling the CO₂ released from the contacting step to liquefy the vapor of the solvent in the CO₂, and removing the solvent to purify the CO₂. 